The field of the invention is water heating systems having automatic control and the present invention is particularly concerned with water and - or steam-heating systems using more than one boiler to economize on fuel.
The state of the art may be ascertained by reference to U.S. Pat. Nos. 1,650,993; 1,786,900; 1,932,269; 2,046,760; 2,168,680; 2,298,847; 2,300,560; 2,480,883; 3,171,596; 3,305,172; 3,351,128 and 3,672,567, the disclosures of which are incorporated herein.
The closest prior art appears to be U.S. Pat. No. 2,480,883 of Schramm, which shows the use of a coal fired boiler and an auxiliary oil fired boiler.
Although a number of the prior art patents disclose the use of an auxiliary boiler for purposes of fuel economy, none of them have the concept of using a smaller auxiliary boiler with a more efficient burner to maintain the water temperature in the main boiler at an elevated value and to generate heat for days in spring and autumn when moderate warmth is required.